To Williams and his family, his ability to recall events — and especially dates — is a regular source of amusement.

But according to one expert, Williams' skill might rank his memory among the best in the world. Doctors are now studying him, and a woman with similar talents, hoping to achieve a deeper understanding of memory.

Williams, a radio anchor in La Crosse, seems to enjoy having his memory tested. Name a date from the last 40 years and, after a few moments, he can typically tell you what he did that day and what was in the news.

How about Nov. 7, 1991?

"Let's see," he mused, gazing into the distance for about five seconds. "That would be around when Magic Johnson announced he had HIV. Yes, a Thursday. There was a big snowstorm here the week before."

He went on to identify correctly some 20 other events, including the birth of the first test-tube baby in 1978, the toxic-gas leak in Bhopal, India, in 1984, and Billie Jean King's victory over Bobby Riggs in tennis' "Battle of the Sexes" in 1973.

"I've always been this way," Williams said. "Growing up, I never really had reason to think I wasn't like everyone else."

So how does he do it?

"You want the Nobel Prize right now? Tell me that answer and I'll publish it," said Dr. James McGaugh, who has studied Williams since last summer. "We don't know. We do know that he carries this information with him, that it's detailed, that it's just there. That's what we want to know — why is it there?"

Williams' brother first contacted McGaugh, a research professor at the University of California, Irvine, after the neurobiologist published a case study of a similar person in the journal Neurocase in 2006.

That woman is in her mid-40s and was identified only by the initials A.J. She told McGaugh whenever she hears a date, memories from that date in previous years flood her mind like a running movie.

The phenomenon, she laments, is "nonstop, uncontrollable and totally exhausting."

"Most have called it a gift, but I call it a burden," she wrote. "I run my entire life through my head every day and it drives me crazy!!!"

McGaugh and his colleagues subjected A.J. to a battery of psychological tests. Given a date at random, she was nearly flawless in recalling the day of the week and what she did that day. The details she provided invariably matched what she had written in diaries decades earlier.

Scientific literature documents people who could memorize a series of 50 to 100 random letters or digits. Another person read a 330-word story twice, then reproduced it nearly verbatim a year later.

But those research subjects remembered meaningless information. What distinguishes Williams and A.J. is their "superior autobiographical memory" — an above-average ability to remember dates and details from their distant past, McGaugh said.

"In subjects we regard as having this ability, they do better than 90 percent on the tests we provide," McGaugh said.

The tests typically involve reproducing personal information that can be corroborated with old scrapbooks, yearbooks and diaries, sources that McGaugh often tries to obtain from family members without the subjects' knowledge.

Other tests involve naming a notable public event and asking for its date, or vice versa.

Williams and A.J. both performed better on topics that interested them. Williams excels at pop-culture trivia such as Academy Award winners, but he stumbles on sports.

A lifelong bachelor and self-described Scrabble addict, he finished second when he appeared on "Jeopardy!" in 1990.

He says he went 5-for-5 on "1984 movies" but tripped up on categories including "snakes" and "words that begin with 'kh'."

Because a person's interest in the information is a key factor in recall ability, some researchers doubt that Williams and A.J. are unique.

"If it's a truly amazing memory that just sucks things up, it shouldn't be based on how interesting something was to you," said Stephen Christman, a neuropsychologist at the University of Toledo in Ohio.

Christman, who wasn't involved in the research, pointed to baseball fanatics who remember obscure statistics because of their passion for the game.

Perhaps, he speculated, A.J. obsesses so much over past events and relives them so frequently in her mind that it's now effortless for her to recall countless dates and events.

The number of people with comparable memory skills has been hard to pin down. After publishing his research with A.J., McGaugh heard from about 50 people claiming they had the same skill or, like Williams' brother, knew someone who might.

Of them, McGaugh and his colleagues have identified a third person — a 50-year-old Ohio man — who shows similar promise.

Ever since pointing his elder brother in McGaugh's direction, Eric Williams, 45, has been recording Brad's adventures for an upcoming documentary.

The movie, to be titled "Unforgettable," is scheduled to be completed later this year.

"The human brain is the most complicated and important machinery in the known universe," McGaugh said. "My aim with this research isn't to cure Alzheimer's. It's to decrease the mystery of this marvelous machinery."